This invention relates to fuel-air mixture feed devices for use with multiple cylinder and multiple carburetor type for motor vehicles, and more particularly to a fuel-air mixture feed device of the type described adapted for use with a motorcycle.
In an engine of high power per unit swept volume, it is necessary to increase suction efficiency and the throat diameter of the Venturi pipe so as to supply a large volume of air to the cylinder of the engine. However, an increase in the throat diameter of the Venturi tube leads to a disadvantage wherein vaporization of the fuel is not carried out satisfactorily at low loads, because the flow velocity of the air passing through the Venturi is reduced and the negative pressure of the air intake does not reach a sufficiently high level to obtain satisfactory atomization of the fuel.
In the case of a fuel-air mixture feed device of the multiple cylinder and single carburetor type which is generally in use in four-wheeled motor vehicles, all the cylinders are interconnected by means of a manifold. This arrangement enables negative pressure of air intake to be increased by the synergystic effect of the negative pressure of air intake into each cylinder, with the result that the performance of the engine at low loads is improved as compared with an engine of the type having a carburetor for each cylinder. However, if this construction is incorporated in a motorcycle engine, it will lead to a reduction in maximum power of the engine due to the facts that a resistance is offered to the air by the elongated manifold and that a sufficiently large volume of suction air to obtain the required air-fuel ratio is unobtainable in the case of a single carburetor engine.
In order to obviate this disadvantage, proposals have been made to use a compound carburetor, mainly with fuel-air mixture feed devices of four-wheeled motor vehicles, which comprises two carburetor bores, one carburetor bore having a smaller Venturi throat diameter and the other carburetor bore having a larger Venturi throat diameter, the carburetor bore of the smaller Venturi throat diameter alone being rendered operative at low loads and the two carburetor bores of the smaller and larger Venturi throat diameters both being rendered operative at high loads. However, the use of this compound carburetor in a fuel-air mixture feed device of the multiple cylinder and multiple carburetor type will have disadvantages in that the carburetor assembly as a whole becomes large in size and the conjointly operating mechanism for the carburetors will become complex in construction. Particularly when the compound carburetors are incorporated in a motorcycle of the multicylinder engine, difficulty will be encountered in designing the mounting of the engine on the motorcycle due to the aforesaid disadvantages.